Bones and Joints The four bones of the shoulder: The humerus is the upper arm bone. This is the "ball" of the shoulder's "ball and socket" joint. The scapula is the flat, triangular bone commonly called the shoulder blade. Prominent areas of the scapula serve as attachment points for many muscles and ligaments. The glenoid is the shallow "socket" on the side of the scapula that receives the 'ball' of the humerus. Together they form the "ball and socket" arrangement of the shoulder. The acromion is the end of the scapular spine. It projects forward to form the top of the shoulder. The coracoid process is a projection towards the front of the scapula and is an attachment site for several muscles and ligaments. The clavicle or collarbone. Although it appears to be straight, it is actually S-shaped when seen from above. The thorax, or rib cage, is an anchor for several muscles and ligaments. Although the ribs do not physically attach to the scapula, the thorax stabilizes and maintains proper positioning of the scapula so that the arm can function to its fullest capacity. Together these four bones form four junctions, or joints: The glenohumeral joint is the main joint of the shoulder. Here, the glenoid on the scapula and the head of the humerus come together to form a ball and socket joint. The fairly flat socket of the glenoid surrounds only 20% - 30% of the humeral head. Because of its poor fit, this joint relies heavily on the surrounding ligaments and muscles for support. The labrum, a ring of fibrocartilage tissue, attaches to the glenoid and deepens the socket to encircle more of the humerus. The acromioclavicular joint, or AC joint, is the bony point on the top of the shoulder. It anchors the scapula to the chest, by connecting the acromion on the scapula to the clavicle, or "collarbone". A thick disk of fibrocartilage acts as a shock absorber between the two bones. The surrounding capsule and ligaments give this joint great stability. The sternoclavicular joint, or SC joint, connects the other end of the clavicle to the sternum, or "breastbone". Like the AC joint, this joint contains a fibrocartilage disk that helps the bones achieve a better fit. It also gets strong support from its joint capsule and surrounding ligaments. The scapulothoracic articulation is the area where the scapula, embedded in muscle, glides over the rib cage. The surrounding muscles and ligaments keep the scapula properly positioned so that the arm can move correctly. Cartilage There are two types of cartilage in the shoulder: Articular cartilage is the shiny white coating that covers the end of the humeral head and lines the inside surface of the glenoid. It has two purposes: To provide a smooth, slick surface for easy movement To be a shock absorber and protect the underlying bone Fibrocartilage is the thick tissue that forms the disks of the AC and SC joints and the labrum, the ring that deepens the glenoid. Fibrocartilage has three roles: To act as a cushion in shock absorption To help stabilize the joint by improving the fit of the bones To act as a spacer and improve contact between the articular cartilage surfaces Ligaments The shoulder relies heavily on ligaments for support. Ligaments attach bone to bone and provide the "static" stability in a joint. Ligaments keep the joint within the normal limits of movement. The glenohumeral ligaments attach in layers from the glenoid labrum to the humerus, forming the joint capsule around the head of the humerus. The coracoacromial ligament is a ligament that spans the bony projections of the coracoid process and the acromion. The coracoclavicular ligaments (CC ligaments) and the acromioclavicular ligament (AC ligament) provide most of the support for the AC joint. Muscles and Tendons Muscles and tendons work together in the shoulder to provide the "dynamic" stability of the shoulder. There are four muscle groups in the shoulder: The rotator cuff muscles are the subscapularis, the supraspinatus, the infraspinatus, and the teres minor. They are the primary stabilizers that hold the "ball" of the humerus to the glenoid "socket". The socket is too shallow to offer much security for the humerus. These four muscles form a "cuff" around the humeral head, securing it firmly in the socket. As its name implies, this group of muscles also rotates the arm. The rotator cuff protects the glenohumeral joint from dislocation, allowing the large muscles that control the shoulder to power the arm with great mobility. The biceps tendon complex also helps to keep the humeral head in the glenoid and flex and rotate the arm. The scapulothoracic muscles attach the scapula to the thorax. Their main function is to stabilize the scapula to allow for proper shoulder motion. The superficial muscles of the shoulder are the large, powerful outer layer of muscles that are important to the overall function of the shoulder. This group includes the deltoid muscle, which covers the rotator cuff muscles. Bursae A bursa is a pillow-like sac filled with a small amount of fluid. Bursae (plural) reduce friction and allow smooth gliding between two firm structures, like bone and tendon or bone and muscle. There are over 50 bursae in the human body; the largest is the subacromial bursa (under the acromion) in the shoulder. The subacromial bursa and the subdeltoid bursa (under the deltoid muscle) are often considered as one structure. This bursa separates the rotator cuff from the acromion and acts as a spacer between them.